An anchored boat packed with sleeping tourists sank early Thursday in Vietnam's scenic Ha Long Bay, killing 12 people from nine countries in the deadliest tour boat accident since the country opened to foreign tourism 25 years ago.

Holidaymakers from the US, Britain, Australia, Japan, Russia, France, Sweden and Switzerland have been confirmed dead, along with a Vietnamese tour guide, the government said. Another nine foreigners and six locals were rescued by other tour boats in the bay's emerald waters.

Ha Long Bay is one of the country's top tourist attractions, drawing more than 5 million visitors a year, many who choose to stay overnight on wooden boats equipped with sleeping cabins.

Survivors reported seeing a wooden plank on their ship ripping away around 5 a.m. as it was anchored for the night, followed by gushing water inundating the boat and quickly pulling it under near Titov island, said Vu Van Thin, chief administrator of Quang Ninh province.

"Crew members tried to stop the water from coming in and alerted the tourists who were sleeping, but the water came in and the boat sank quickly," he said. "All of the 12 people who died were in the cabins."

There were 27 people, including six crew members, aboard the boat and all have been accounted for, Thin said. The vessel, which is owned by Truong Hai Co., was anchored alongside dozens of other cruise boats and weather conditions were calm at the time of the incident.

The dead have been sent to Bai Chay Hospital for identification, where survivors received treatment for minor injuries, said Ngo Van Hung, director of Ha Long Bay's management board.

The official Vietnam News Agency published the victims' names and ages, most of them aged 20 to 25. They include a Briton, two Americans, one Japanese, one French, two Swedes, two Russians, one Swiss and one person of Vietnamese origin living in Australia, according to the government.

"This is a very rare and very unfortunate accident," said Foreign Ministry spokeswoman Nguyen Phuong Nga. "I think after this incident, the People's Committee of Quang Ninh province and tourist companies will have to seriously review the cause of this accident to work out measures to ensure the highest safety for tourists in Ha Long Bay."

The hospital's deputy director, Giang Quoc Duy, said survivors "were in a panic."

"They were given first aid treatment and have already returned to their hotels," he said.

Vietnam's foreign ministry confirmed the survivors as two Danes, one German, two Italians, one American, one Australian, one French and one Swiss.

Ha Long Bay, a UNESCO World Heritage site dotted with limestone formations, is located near the Chinese border in the Gulf of Tonkin about three hours east of the capital, Hanoi.

More than 100 cruise boats are licensed to offer overnight service there, and last year the area received 5.4 million visitors, nearly half of them foreigners, according to government websites.

The bay has seen boats go down in the past. In 2009, a tour boat sank during a storm, killing five, including three foreign vacationers. In 2006, a powerful wind storm capsized several boats, killing 13 people, though no tourists were among the dead. In 2002, strong winds capsized two tourist boats, killing several foreigners.